Electric resistor



Feb. 20, 1951 s. GILMAN ET AL 2,542,308

ELECTRIC RESISTOR Filed Dec. 12, 1945 WITNESSES:

I Patented-Feb, 1951 ELECTRIC RESISTOR Samuel Gilman, Baltimore, Md.,and Jerald E.

Hill, Forest Hills. Pa...

house Electric Corpor assignors to Westingation, East Pittsburgh,

Pa a corporation of Pennsylvania ApplicationDecember 12, 1945, SerialNo. 834,551

coating of a carbonaceous material on an insulating sheet also have beenmade use of. However the resistance cards are subject to widefluctuations in resistivity even in various sections of the same card.The resinous dielectric materials vary considerably amongst themselvesas to the losses produced due to the phys cal nonuniformity thereof. Allof these expe ients are limited in attenuation to very small values,usually less than 10 decibels for members six inches in length. Largerattenuations have been only secured by using extremely long pieces ofeither resistor cards or lossy dielectric material.

Furthermore, changes in atmospheric humidity and the relatively highdegree of moisture absorption of these prior art attenuators causerelatively great changes in the attenuation with variations in theambient atmosphere. Constancy of the ultra-high frequency circuits hasbeen, therefore, difllcult to obtain by their use.

The object of this invention is to provide for a molded member capableof use as an attenuator wherein the attenuation may be predeterminedwithin a wide range of values for given physical dimensions.

A further object of the invention is to provide a molded member suitablefor use as an attenuator, which member is so relatively impervious tomoisture that its properties do not vary appreciably under normalconditions of use.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description anddrawing, in which;

Figure 1 is a perspective view of an attenuator constructed according tothis invention;

Fig. 2 is a perspective view of another modi fication of an attenuator;and

Fig. 3 is a perspective view of a shielding member likewise constructedin accordance with the invention.

We have discovered that attenuators having new and unusual propertiesmay be prepared by combining predetermined proportions of finely dividedgraphite or other energy absorbing solid with a compLtely polymerlzablefluid resin composition composed of (A) 5 to 95 parts by weight of aliquid monomer having the group 8 Claims. (Cl. 201-83) and of (B) fromto 5 parts by weight of the reaction product of an ethylene alpha-betadicarboxylic acid or anhydride thereof with castor oil alone or combinedwith another vegetable oil and polymerizing the resin composition to athermoset state. The resin composition carrying a predetermined amountof graphite dispersed therein is completely polymerizable and may becast or otherwise molded into any predetermined shape wherein thecomponents (A) and (B) copolymerize completely without the necessity ofevaporating volatile components. This provides that the members soprepared are homogeneous and free from voids, pores or other flaws. Forthe purpose of this invention this homogeneity and freedom from voids.is critical. The thermoset resinous material has outstanding moistureresistance. This latter property is important since it assureseonstancyof attenuation. By varying the proportions of the liquid monomer to thereaction product, the members prepared may be either soft and rubbery ormay be quite hard whereby they are readily machinable in conventionalmetal working machines. In all cases, the final product is a thermosetresin which does not fuse or liquefy even at high temperatures short ofcarbonization.

The reaction product (B) employed in producing the fluid resincomposition may be prepared by reacting castor oil alone or castor oilcombined with another vegetable oil. Drying oils such as linseed oil,tung oil, hempseed oil], poppy seed oil, sunflower oil, walnut oil, andoiticica oil; semidrying oils such, for example, as corn oil, cottonseedoil, pumpkin seed oil, sesame oil and soybean oil; and nondrying oilssuch as peanut oil and olive oil may be admixed with the castor oil. Ifsoft and flexible resinous products are desired, a nondrying oil such aspeanut oil is admixed as the major ingredient with the castor oil inminor proportion. In any event, a substantial proportion of the mixtureof oils should be castor oil. As the lower limit, about 25% of the totalamount of the oils should be castor oil.

' Examples of suitable ethylene alpha-beta dicarboxylic acids andanhydrides thereof to be reacted with the castor oil are maleic acid andmaleic anhydride. Citraconic anhydride, chlormaleic anhydride and theiracids are other examples of suitable compounds for reaction with theoils. The reaction with the oil is most conveniently carried out byemploying the anhydrides. though by use of somewhat higher temperaturesduring the reaction, and in some cases the addition of a suitablecatalyst, the corresponding acids may be employed.

It is desirable to react the castor oil, with or without anothervegetable oil, and the ethylene alpha-beta dicarbcxylic acid oranhydride to be obtained in the final resinous product.

produce the half ester of the castor oil. For example, when castor oilis reacted with maleic anhydride, castor oil maleate is formed. One, twoor three molecules of maleic anhydride can be reacted with each moleculeofcastor oil "since the castor oil contains three ricinoleic acidradicals, each of which has a hydroxy group at which the esteriflcationof the acid anhydride may occur. The full half ester composed of threemolecules of maleic anhydride per castor oil molecule is quite desirablefor some forms of the invention since the maximum degree ofcross-linkage will, The full half ester produced by reacting three molsof maleic anhydride with one mol of castor oil is believed to have thefollowin composition:

The monomaleate half ester of castor oil and thedimaleate of castor oilmay also be used in carrying. out the invention just as successfullyasthe full half ester. Iifigheral, 'from three to ten parts, by weight, ofmaleic anhydride for every thirty parts, by weight, of castor oil willproduce a reaction product having satisfactory characteristics. I Thenonhydroxylated oil component of the oil mixture probably reacts with anunsaturated dibasic anhydride, such as maleic anhydride, by esterinterchange and the Diels-Adler addition reaction, depending on themolar proportions of the ethylene alpha-beta dicarboxylic acid as wellas the conditions of the reaction. The reaction product secured willvary to some extent on these conditions of reaction. However, at leastone mol of the ethylene alpha-beta dicarboxylic acid, such as maleicanhydride, should be present for each mol of the vegetable oil in themixture. The reaction products of the castor oil, with or without othervegetable oils, with the ethylene alpha-beta dicarboxylic acids oranhydrides are generally syrupy or soft gummy substances. They may bedissolved in liquid monomers having the group H2CC in the proportions offrom parts to 95 parts by weight of liquid monomers and 90 parts to 5parts by weight of the reaction product.

In forming a liquid composition, various liquid monomers having thegroup H2C=C have been found satisfactory as solvents and co-react-antsfor the reaction product of the maleic anhydride and mixed acid esters.Monostyrene is one highly eiTective copolymerizable solvent. Nuclearlychlorinated and monoalkyl nuclear substituted styrenes are similarlyeffective for the practice of the invention; paramethyl styrene andparachlor styrene being typical examples. Other monomers having thegroup H2C=C such, for example, as alpha-methyl styrene, alpha methylpara methyl styrene, vinyl acetate and other vinyl esters, methyl vinylketone, acrylic nitrile, methyl methacrylate and allyl esters, such asdiallyl phthalate have been used to dissolve and to copolymerize withthe ester reaction product with successful results.

The use of higher proportions of the liquid monomer (A), such asstyrene, to the reaction product (B) results in a harder final resinproduct. The and higher styrene copolymers are quite hard, whereas thecopolymers having 75 and less styrene are more flexible and elastic. Forservice involving shock and stresses, the use of the copolymer having25% or more of the reaction product (B) of the mixture of fatty acidesters and dibasic acid is, therefore, suggested.

In order to prevent spontaneous polymerization and undue thickening ofthe solution in the liquid vinyl monomer, it is desirable to incorporatea small quantity of inhibitor in the solution.

Example No. I

A reaction product (13) composed of:

parts by weight of castor oil 30 parts by weight of maleic anhydride wasprepared by heating the mixture at C. for several hours to produce acastor oil maleate of a molasses-like consistency. About 15 parts byweight of the cooled castor oil maleate was dissolved in 85 parts byweight of monomeric styrene plus 0.02% of hydroquinone to inhibitpremature polymerization. A resinous solution of the consistency of thinoil was so produced. With 1% benzoyl peroxide, sections having athickness of half an inch were cast and upon heating for four hours at-C. formed excellent thermoset members.

In order to enhance the thixotropic properties of the solution ofExample I, 65 parts by weight of the solution was admixed in anevacuated flask with 35 parts by weight of 325 mesh mica and 1% ofbenzoyl peroxide catalyst based on the weight of the resinouscomponents. A thick, goldenbrown resinous material was produced by theprocess. When heated at 125 C. for several hours, the material with themica thermoset into a solid void-free body.

The castor oil-maleate-styrene thermoset resin has an exceptionally flathardness-temperature curve. Durometer measurements of one sample at 28C. gave values of 80 while at 100 C. the value was '70. This relativelysmall change in hardness over this range is a particularly valuablefeature.

Oils and many other petroleum products exert no solvent effect on thisfamily of thermoset resins.

Example No. II

A reaction product composed of:

61 parts by weight of linseed oil 15.8 parts by weight of castor oil23.2 parts by weight maleic anhydride was produced by heatingthe mixturefor eight hours at C. to 200 C. 75 parts of this resin were dissolved in25 parts of monomeric styrene and 0.03% hydroquinone inhibitor and 1% byweight of benzoyl peroxide catalyst was added. Graphite of 325 meshfineness was added and cast members were readily obtained by heating attemperatures of from 65 C. to 150 C.

Example No. [II

A mixture of parts by weight .of castor oil 60 parts of peanut oil 20parts of linseed oil parts of maleic anhydride was heated at atemperature of 175 C., for eight hours to a thick syrupy state. Thereaction product was dissolved in 30 part by weight of monostyrene to 70parts by weight of the linseed oilcastor oil-peanut oil maleate and 1%benzoyl peroxide was added. The presence of the peanut oil preventedundue oxidation of the surface as compared to that produced on thelinseed oilcastor oil resin of Example No. II. The proportion of peanutoil may be increased or decreased to meet requirements.

For preparing an attenuator member the resin of Example I composedof 15parts of castor oil maleate and 85 parts of monostyrene by weight andcarrying of mica dust was molded by casting into a mold. The resinousmaterial was polymerized by heat treating at a temperature of 135 C. forseveral hours, and then machined into the shape of member Ill shown inFig, 1 of the drawing. The dimension between the faces 12 wasapproximately one inch. The attenuation of the member was 1.25 decibelsat 100 megacycles.

The resin of Example I, but containing 35% castor oil maleate and 65%monostyrene, was placed in a flask and 25% by weight of finely dividedgraphite was added thereto and admixed while under a vacuum. Thethoroughly mixed resin and graphite composition was then cast in a moldand after polymerization was machined to the shape of member Ill of Fig.1 of the drawing with six inches between the tips l6l8. The attentionwas 31.6 db.; the dielectric constant at 215 megacycles was 18.8. Whenthis last mentioned resin was admixed with 43.2% graphite in anevacuated flask and then molded and machined into a thermoset member,the attenuation for a member having four inches between tips lG-IB wasgreater than 80 decibels. A spectrum analyzer showed an attenuation ofapproximately 90 decibels. The dielectric constant at 195 megacycles was30.2.

The resin of Example I was combined with various amounts of finelydivided graphite with the following results when tested with the sameultra-high frequency wave band:

Example No. V

Graphite Per cent wave lengths, whereas the resistor cards as employedheretotfore have the reverse property.

While graphite of a fineness of 200 mesh and finer embodied in thethermoset resin herein described has produced excellent results, otherenergy absorbing conducting-powders may be added to replace a part orall of the graphite. Lampblack, wood chars and similar carbon materialsmay be incorporated in the resinous composition. Titanium dioxide hasbeen combined with the graphite in preparing attenuators which gaveexcellent results in service. manganin and similar metals may beincorporated in the resin composition.

Referring to Fig. 1 of the drawing, there is illustrated a typicalattenuator member I 0 having two parallel faces I! whose distancedepends on the wave guide height-one inch being a common dimension. Thethickness of the member may be of the order of /2 inch. The body of themember l0 slopes inwardly toward the center along sides It to a narrowcentral portion. The distance between the tips [6-48 determinestlie,eifectiveness of the attenuator.

Figure 2 of the drawing illustrates a difierent form of attenuator 30which has given good results. The attenuator 30 has a dimension of aboutone inch betweensldes 32-42, though this distance may varyiii/accordance with the wave guide width. The distance between faces34-34 may be as little as /2 inch and as much as 4 inches and longer.The rectangular projections -36, -disposed lengthwise of th wave guide,may

be cubes with sides of /2 inch dimension.

The molded resin composition and graphite members of this invention maybe employed for numerous other applications. Various shieldin materialsto prevent radio frequency power and noise leakages, coaxial linattenuators, shielding around joints in coaxial lines and wave guidesmay be produced from the compositions described herein. The fluidcompositions may be molded in place for either wave guide or coaxialline attenuators.

Referring to Fig. 3 of the drawing, there is illustrated the cylindricalshielding member 20 composed of a molded body 22 with an internal bore24 to accommodate an electrical conductor. The member 20 may be preparedfrom any of the resinous compositions herein described with proportionsof finely divided graphite, or other energy absorbing material.

Due to the unusual moisture resistance of the thermoset resinouscompositions described herein, the attenuators and other ultra-highfrequency members prepared therefrom will not be subject to anysignificant variation in properties due to variations of atmospherichumidity or even by contact with water. The attenuation will berelatively constant regardless of the age of the attenuator Or theconditions of operation. These features are not obtainable with otherresins available to the art and enable a unique product to be produced.

The resin compositions described herein, particularly those having over75% of a monomer, such as monostyrene, have closely similar low lossvalues and, therefore, ordinary variations in the proportions of theresin components to one another do not cause any appreciable change inattenuation. The proportion of graphite or other conducting powder tothe resin will be the critical determining factor of the characteristicsof the attenuator. This feature enables the preparation of attenuatorsthat are mechanically sim- Finely divided iron,

7 lie: but may have a value of attenuation selected from a wide range. Aclose control of the graphite or other conducting powder within themolded members will control the degree of attenuation within reasonablyclose limits.

Since certain changes may be made in theabove invention and differentembodiments of the invention may be made without departing from thescope thereof, it is intended that all matter contained in the abovedescribed disclosure shall be interpreted as illustrative and not in alimiting sense.

We claim as our invention:

1. A member suitable for use as an attenuator in a circuit carrying anultra-high frequency electrical current comprising, in combination, amolded member of a thickness of the order of half an inch composed ofthermoset resin having highly electrically insulating properties, theresin derived by reacting from to 95 parts by weight of a monomer havingthe group H2C=C and from 95 to 5 parts by weight of the ester reactionproduct of castor oil and an acidic compound selected from the groupconsisting of ethylene alpha-beta dicarboxylic acids and theiranhydrides, and from 2% to 50% of the weight of the member composed offinely divided electrically conducting material distributed throughoutthe resin.

2. A member suitable for use as an attenuator in a circuit carrying anultra-high frequency electrigaiwurrent comprising, in combination, a

molded member of a thickness of the ,orderof half an inchcomposedTfFiermoset resin having highly electrically insulatingproperties, the resin derived by reacting from 5 to 95 parts by weightof a monomer having the group HzC=C and from 95 to 5 parts by weight ofthe reaction product of castor oil and another vegetable oil with anacidic compound selected from the group consisting of ethylenealpha-beta dicarboxylic acids and their anhydrides, and from 2% to 50%of the weight of the member composed of finely divided graphitedistributed throughout the resin.

3. A member suitable for use as an attenuator in a circuit carrying anultra-high frequency electrical current comprising, in connection, amolded member of a thickness of the order of half an inch composed ofthermoset resin body derived by reacting from 5 to 95 parts by weight ofa monomer having the group H2C=C and from 95 to 5 parts by weight of theester reaction product of castor oil and maleic anhydride, and from 2%to 50% of the weight of the member composed of finel divided graphitedistributed throughout the resin.

4. A member suitable for use as an attenuator in a circuit carrying anultra-high frequency electrical current comprising, in combination, amolded member of a thickness of at least inch composed of thermosetresin body derived by reacting i'rom 5 to parts by weight of a monomerhaving the group H2C=C and from 95 to 5 parts by weight of the esterreaction product of castor oil and an acidic compound selected from thegroup consisting of ethylen alpha-beta dicarboxylic acids and theiranhydrides, finely divided graphite distributed in the resin, and asubstantial amount of mica powder embodied in the resin.

5. A member suitable for use as an attenuator in a circuit carrying anultra-high frequenc elec trical current comprising, in combination, amolded member of a thickness of the order of half an inch composed ofthermoset resin body derived by reacting from 5 to 95 parts by weight ofmonostyrene and from 95 to 5 parts by weight of the ester reactionproduct of castoroil and an acidic compound selected from the groupconsisting of ethylene alpha-beta dicarboxylic acids and theiranhydrides, and from 2% to 50% of the weight of the member of finelydivided graphite distributed in the resin.

SAMUEL GILMAN. JERALD E. HILL.

REFERENCES CITED "recruiting references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Book: Microwave TransmissionDesign Data, by T. Moreno, published May 1944, by Sperry Gyroscope Co.,Inc., Manhattan Bridge Plaza, Brooklyn 1, New York, publication No.25-80, page 210.

1. A MEMBER SUITABLE FOR USE AS AN ATTENUATOR IN A CIRCUIT CARRYING ANULTRA-HIGH FREQUENCY ELECTRICAL CURRENT COMPRISING, IN COMBINATION, AMOLDED MEMBER OF A THICKNESS OF THE ORDER OF HALF AN INCH COMPOSED OFTHERMOSET RESIN HAVING HIGHLY ELECTRICALLY INSULATING PROPERTIES, THERESIN DERIVED BY REACTING FROM 5 TO 95 PARTS BY WEIGHT OF A MONOMERHAVING THE GROUP H2C=C< AND FROM 95 TO 5 PARTS BY WEIGHT OF THE ESTERREACTION PRODUCT OF CASTOR OIL AND AN ACIDIC COMPOUND SELECTED FROM THEGROUP CONSISTING OF ETHYLENE ALPHA-BETA DICARBOXYLIC ACIDS AND THEIRANHYDRIDES, AND FROM 2% TO 50% OF THE WEIGHT OF THE MEMBER COMPOSED OFFINELY DIVIDED ELECTRICALLY CONDUCTING MATERIAL DISTRIBUTED THROUGHOUTTHE RESIN.